CIA Director George Tenet, who has been under attack over the intelligence used to justify the Iraq invasion and warnings before the September 11 attacks, resigned on Thursday, the US administration announced.
President Bush told a hastily convened news conference just before he left for a major trip to Europe that Tenet was leaving for "personal reasons".
"Today, George Tenet, the director of the CIA submitted a letter of resignation," Bush said on the South Lawn of the White House. "I accepted his letter."
The president said he had met with Tenet on Wednesday.
"I had a good visit with him. He told me he was resigning for personal reasons. I told him I'm sorry he's leaving. He's done a superb job on behalf of the American people," said Bush.
Deputy Central Intelligence Agency director John McLaughlin will take over in charge of the world's most powerful spy network in mid-July, when Tenet's resignation becomes effective.
No permanent replacement is likely to be announced until after the November 2 presidential election, analysts said.
Any new nomination before the election would require Congress confirmation hearings that could reopen controversies over Iraq and the 2001 attacks on New York and Washington.
The son of Greek immigrants, Tenet grew up in Queens, New York.
Tenet was appointed in July 1997 by President Bill Clinton but remained close to Republican boss President George W. Bush after the 2000 election. He was the second longest serving head of the world's most powerful spy agency.